Focusing a DSLR on a dob

[Jorge (Jorge)]

Ok, so I'm sure to be told this has been covered a trilion times but just once more for the kid at the back of the class , may I ask for guidance ?...(perhaps AP intervention...?)
I am using a Canon 60D on 14" autotracking goto dob (fl 1650mm, f/4.6). I attach the camera to the scope with a Vixen T-ring and add a MPCC to minimise coma. I aim for extremely short exposures and use only manual settings...so...despite trying a 1.25" 2x Barlow, I could not focus using live view.
Have I completely missed the point ...(pardon the pun)? The focus point "appears" to be still inside the focuser tube, even with the Barlow lens attached. Should I be trying a focal extender or a focal reducer? Should I be focusing only using provided software, rather than at the scope? (I was hoping not to drag a laptop around).
i would be glad for your guidance, thanks in advance.
Jorge.

[ZeroID (Brent)]

Mechanically speaking you have to able to get focus at the scope. If you imagine the DSLR is the eyepiece you have to be focussed there.
Sounds like your scope focal point is inside ( or possibly outside ) the range of the focusser\DSLR movements even with any other optical device. Knowing sods law it will probably be just inside the range your setup allows.
Either you can measure the exact stated focal length up the tube and out to the focusser from the diag to see where it should theoretically be.
Or you can do what I do when building. Take the whole focusser off and do a test with hand held camera.
It possibly sounds like you may have no alternative but to lift the primary mirror up the OTA to get the sweet spot.

[rustigsmed (Russell)]

How far off do you think the focus is Jorge?
You might have to buy 3 longer screw to push the primary mirror up or get a low profile focuser. also maybe try it without the MPCC to see if you can reach focus?

[jjjnettie (Jeanette)]

Which goto dob do you have? When I image using my 10"Skywatcher Goto, I just don't fully extend the top half, stopping approx 1" short. That will allow you to achieve focus easily.

[Jorge (Jorge)]

The scope is the Orion 14" goto auto tracking dob. Though it is a truss tube design, the truss tubes are not collapsible. I slowly raked the focuser all the way to the top and down again...no cookie; I've tried the camera with no MPCC with no success.
I will remove the focuser and try that first. Both the focuser and T-ring are already low profile. Moving the mirror up by way of longer screws might be my next attempt.
I will keep you updated .
Jorge.

[knightrider]

Originally Posted by jjjnettie

Which goto dob do you have? When I image using my 10"Skywatcher Goto, I just don't fully extend the top half, stopping approx 1" short. That will allow you to achieve focus easily.

I can keep my 10" collapsible fully extended when I take photos. It reaches focus easily with room to move on either side of the focal point. I use it at prime focus....am I doing something different?

[jjjnettie (Jeanette)]

I'm not sure. I use a pretty low profile adaptor and it won't achieve focus. Do you screw the T ring straight to the focuser?

[knightrider]

Originally Posted by jjjnettie

I'm not sure. I use a pretty low profile adaptor and it won't achieve focus. Do you screw the T ring straight to the focuser?

Actually I use a 2" nose piece/T-Adapter that I screw onto the canon T-Ring. I just insert this into the 2" skywatcher adapter where the eyepiece would normally go.

[rustigsmed (Russell)]

My Sky Watcher 12" also allows focus no worries. I think its just the subtle differences between makes.